This invention relates to spectroscopic ellipsometers and methods of ellipsometry.
In one class of spectroscopic ellipsometer, an optical system focuses light from a light source through a monochromator, a polarization-state generator, a sample and an analyzer to determine the polarization state of the light caused by the reflection from the sample. The resulting light pattern is detected by a photodetector to provide an analog electrical signal that indicates the polarization state of the light. In some ellipsometers, the analog electrical signal is converted to a digital signal. The position of the sample and detector is usually manually adjustable to obtain the proper angle of impinging light.
In a prior art type of spectroscopic ellipsometer of this class, the optical system includes one or more lens systems, mirror systems or a combination of the two types of systems to focus the light from the light source through a monochromator slit onto the diffraction grating. The grating and mirrors are prealigned and calibrated in the monochromator. The light reflected from the sample, after being affected by an analyzer, is detected by a photomultiplier tube, and then converted to an electrical signal. The electrical signal is directly digitized by an analog-to-digital converter.
The prior art systems of this type have the disadvantages of being unduly complex, expensive and time-consuming to adjust. Part of the complexity occurs because photomultiplier tubes are used to obtain increased sensitivity instead of using diode detectors. However, simple, inexpensive diode detectors usually have too poor a signal-to-noise ratio with the intensity of light provided by the prior art spectroscopic ellipsometers.